Dental models, in which the individual teeth can be removed, are needed for producing crowns, bridges, and tooth prostheses. In order to form such replacement parts to fit precisely into the model, the model parts must be made removable, which means that, after sawing apart the originally cast model, the model parts must each be mounted on a folded pin which fits into a base plate. Starting from the original jaw impression used to cast the model, the process of correctly positioning the holding pins, whose locations differ as widely as the positions of a particular tooth in different people's jaws, for the respective models, has heretofore been very labor-intensive.
For mounting these pins, one has heretofore used so-called pin sets. These have an X-Y movable stage on whose upper side is disposed the receptacle containing the jaw impression, and whose lower side supports the base plate. A fixed datum or alignment or sounding shaft is located above the jaw impression and corresponds to a fixed drill located under the base plate and having a shank which is aligned with the shank of the sounding shaft. The X-Y movable stage is moved around in such a way that the sounding shaft is directed to the positions where one wishes to later place a pin for mounting a model part. Upon reaching the desred positions, the X-Y movable stage is depressed, so that the drill makes a corresponding hole in the base plate. This process must be painstakingly repeated a number of times corresponding to the number of removable parts needed in the model. Conventionally, each tooth root or removable part in the models requires two pins, so that the part can not rotate when mounted in the base plate.
Finally, pins are inserted into the finished, perforated base plate, the jaw impression is filled with plaster or similar molding material, and the base plate with protruding pins is lowered onto the molding material, taking care that the base plate is exactly aligned, so that where the pins actually penetrate the plaster or other molding material is at the desired positions.
The disadvantages of this conventional procedure are, first of all, that the pin sets are very expensive. Aside from that, during drilling it often happens that the stage moves or the base plate becomes canted, so that the relatively thin drilling bit breaks off. Beyond that, the required procedure with the conventional pin sets is relatively involved, and the precisely undertaken feeling-out of the positions for the pins sometimes comes to naught, because the alignment of the pin-equipped base plate over the jaw impression filled with molding material requires a very precise eye. Finally, the time required for the conventional procedure is very high.